Precipitation-hardenable stainless steel

ABSTRACT

Martensitic chromium-nickel stainless steel of great strength in the age-hardened condition, and of good ductility and toughness. The steel contains about 10.5 percent to about 13.25 percent chromium, about 7.5 percent to about 9.5 percent nickel, about 1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5 percent copper, about 1 percent to about 2 percent aluminum, and remainder substantially iron. The carbon and nitrogen contents are maintained in critically low amount, the former not exceeding about 0.05 percent and the latter not exceeding 0.015 percent. Cobalt up to about 2 percent may be partially substituted for nickel. There may be added columbium up to about 0.3 percent and/or titanium up to about 0.15 percent.

[451 Apr. 25, 1972 [54] PRECIPlTATION-HARDENABLE STAINLESS STEEL WilliamC. Clarke, Jr., Baltimore, Md.

Armco Steel Corporation, Middletown, Ohio [22] Filed: Mar. 6, 1969 [21]Appl.No.: 805,039

[72] Inventor:

[73] Assignee:

2,614,921 10/1952 Tanczyn 75/125 3,083,095 3/1963 Tanczyn .75/1253,362,813 1/1968 Ziolkowski ..75/124 3,408,178 6/1967 Myers ..75/1243,152,934 10/1964 Lula ..75/125 3,278,298 10/1966 Perry..... ..75/128 W3,347,663 10/1967 Bieber ..75/125 Primary Examiner-Hyland BizotAttorney-John Howard Joynt [57] ABSTRACT Martensitic chromium-nickelstainless steel of great strength in the age-hardened condition, and ofgood ductility and toughness. The steel contains about 10.5 percent toabout 13.25 percent chromium, about 7.5 percent to about 9.5 percentnickel, about 1 percent to about 2.5 percent molybdenum, about 1 percentto about 2.5 percent copper, about 1 percent to about 2 percentaluminum, and remainder substantially iron. The carbon and nitrogencontents are maintained in critically low amount, the former notexceeding about 0.05 percent and the latter not exceeding 0.015 percent.Cobalt up to about 2 percent may be partially substituted for nickel.There may be added columbium up to about 0.3 percent and/or titanium upto about 0.15 percent.

13 Claims, No Drawings PRECIPITATION-HARDENABLE STAINLESS STEEL As amatter of introduction, my invention is concernedwith thechromium-nickel stainless steels, more especially those which arehardenable by simple heat-treatment. More particularly, the concern iswith the martensitic chromium-nickel stainless steels which are hardenedby simple-aging treatment at comparatively low temperatures.

One of theobjects of my invention is the provision ofa chromium-nickelstainless steel which not only works well in the mill, as by rolling,drawing, and forging, but which in the form of rolled and drawn productsreadily lends itself to a variety of forming and fabricating operations,such as spinning, upsetting, machining, threading, and the like.

Another object is the provision of a martensitic chromiumnickelstainless steel which readily lendsitself to hardening and strengtheningby simple heat-treatment and yet retains good ductility and toughness inthe hardened and strengthened condition.

A further object of the invention is the provision of a martensiticchromium-nickel stainless steel and various formed,

and fabricated articles fashioned thereof, such as aircraft landinggear, structural parts, fasteners, andthe like, enjoying a high ratio ofstrength-to-weight and a good combination of strength with resistance toshock and impact;

Other objects of my invention in part will become. apparent during thecourse of the description which follows, and in part more particularlypointed to.

My invention, then, resides in the combination of elements, in themixture of ingredients, and in the relation between the same, all asdescribed herein, the scope of the application of which is moreparticularly set out in the claims at the end of the specification.

BACKGROUND OF THE INVENTION In order to gain a better understanding ofcertain features of my invention, it may be well to noteat this pointthat thenumber of grades of stainless steel now available is legion.Perhaps the best known are the austenitic chromium-nickel grades such,for example, as the AISI Types 301 and 302; the former containing about17 percent chromium, about'7'percent nickel, with remainder iron; andthe latter about 18'percent chromium, about 9 percent nickel, andremainder-iron. But these steels, unfortunately, harden significantlyduring a cold-working operation. The A181 Type 305, however, containingabout 18 percent chromium, about 11 percent nickel, and remainder iron,enjoys a substantially lower work-hardening rate. But none of thesesteels may be hardened by heattreatment.

There are available, however, more'sophisticated grades ofchromium-nickel stainless steel which, while enjoying-mostof' thebeneficial corrosion-resisting properties of the chromiumnickel grades,and which readily lend themselves to working,

and forming into a variety of products, may be hardened through agingtreatment. I refer to the chromium-nickel stainless steel described inthe Tanczyn U.S. Pat. No. 3,376,780 of Apr. 9, 1968. That steeltypically containsabout 15 to 18 percent chromium, about 7 to 10 percentnickel, about 2 to percent copper, about 0.75 to 1.50 percent aluminum,and

remainder iron. Where desired, molybdenum may be present in amounts upto 5 percent as a partial substitute for chromium. That steel issemi-austenitic in the solution-treated condition. Andwhilecharacterized by a combination of many highly desirable properties, thesteel in large section, thatis, plate, bar or other products exceedingan inch or two in thickness, does not readily lend itself to hardening,for hardening best is achieved by a combination of substantial, or evendrastic, cold-reduction followed by heat-treatment. Bar, plate, forgingsand other products of significant dimension, where as a result of sizeor other consideration, cold-reduction may not be had, are notavailable, then, in the hardened and strengthened condition.

Also Irefer to the further chromium-nickel stainless steel described inthe copending Clarke-Perry application Ser. No.

585,298, filed Oct. 10, 1966. That steel typically contains about 13percent chromium, 8 percent nickel, 2 percent molybdenum, 1 percentaluminum, with critically low amounts of residual elements, andremainder iron. And while that steel lends itself to hardening byheat-treatment, the

SUMMARY OF THE INVENTlON Turning now more especially to the practice ofmy invention, I provide a chromium-nickel stainless steel whichessentially consists of the five ingredients chromium, nickel,molybdenum, copper and aluminum in particular and critical amount, withcritically controlled amounts of the further ingredients carbon andnitrogen which commonly are found in all stainless steels. In my steel,best results are had by including in thecomposition the furtheringredient columbium, this in small and'critical amount, with titaniumpartially substituted forthe columbium. Where desired, the ingredientcobalt may be partially substituted for the ingredient nickel. The steelis martensitic in the solution-treated condition.

More especially, in my steel chromium is present in the amount of about10.5 or 1 1.5 percent to about 13.25 percent and particularly to about13 percent, nickel in the amount of 7.5 percent to about 9 percent oreven to about 9.5 percent and more especially about 8 percent to about 9percent (or about 8 percent to about 9.5 percent nickel with about 10.5percent to about 12.5 percent chromium), molybdenum in the amount'ofabout 1 percent or about 1.3 percent to about 2.25 percent or about 2.5percent, copper in the amount of about 1 percent to about 2.5 percent,and aluminum inth'e amount of about 1 percent to about 2 percentandparticularly about 1.1 percent-to about 1.4 percent or even to about1.8 percent. For a best combination of results, as more particularlydescribed below, these several ingredients are maintained in morelimited amount.

While the ingredient columbium is not essential to the steel of myinvention, certain benefits are had by its presence in amounts up toabout 0.3 percent, particularly in the amount of about 0.1 percent toabout 0.3 percent. Where desired, titanium may be partially substitutedfor columbium, this only up to the amount of 0.15 percent titanium,however; any greater amount of titanium is found to cause a loss instrength; 1 attribute that loss of strength to the apparent difficultyof the steel taking into solution greater amounts of titanium in theface of the high aluminum content present.

As indicated above, cobalt may be partially substituted for some of thenickel, but this only up toabout 2 percent cobalt, for best resultsabout 1 percent to about 2 percent cobalt, the nickel content beingdecreased with the cobalt addition, but only down to about 6 or 6.5percent. At least 6 percent nickel, even with cobalt and copper present,is required in the steel of my invention in order to assure substantialfreedom from delta-ferrite.

As indicated above, the further and commonly present ingredients carbonand nitrogen are maintained in critically low amount, the carbon notexceeding about 0.05 percent max. and the nitrogen not exceeding 0.015percent max. For a best combination of properties nitrogen should notexceed 0.01

percent or even:0.0l0 or 0.007 percent. The nitrogen content isespecially critical. Ifindthat .withan excessive nitrogenc'on-i tentthere is a'loss offracture toughness.

Actually, I find a certain amount of carbon beneficial to the steel ofmy invention, this as an aid in assuring a desired freedom from anysignificant amount of delta-ferrite, the delta-ferrite being preservedin amount less than 3 percent by volume, for with higher contents thereis a loss of strength, particularly in transverse direction. Carbon,then, for best results is employed in the amount of about 0.025 percentto about 0.045 percent, or more broadly, about 0.02 percent to about0.05 percent.

The further ingredients manganese, silicon, phosphorus and sulphur alsoare maintained low. The manganese and silicon are maintained at valueseach not exceeding 0.10 percent, the phosphorus not exceeding about0.010 percent, and the sulphur not exceeding about 0.005 percent. Theremainder of the composition, of course, is substantially all iron.

My steel preferably is melted in the vacuum furnace in order to assurecleanliness and freedom from oxide inclusions or, indeed, othercontaminants. A single induction vacuum melting operation ordinarily issufficient. 1 find, however, that a superior ingot is had, with soundcenter and minumum waste, by employing a double vacuum treatment, thatis, an initial vacuum melting in the induction furnace followed byconsumable electrode vacuum remelting. The ingots had are clean, sound,and free of hydrogen embrittlement.

The metal works well in the hot-mill in converting ingot to bloom,billet and the like. Moreover, it works well in further conversion tohot-rolled and cold-rolled products, such as plate, sheet and strip,bars, rod, wire and special shapes. These several mill products aresuited to fabrication, as by machining, threading, cold-heading, and thelike, as in the production of threaded fasteners and aircraft parts,particularly landing gear, where a high ratio of strength-to-weight isdesired. As noted above, the steel is martensitic in thesolution-treated condition.

Hardening of my steel and products fashioned therefrom is had merely byheating at aging temperatures, say 900 to 1,050" F., from thesolution-treated condition (heating at some l,400to l,750 F. andquenching), as appears more fully hereinafter. Best and most uniformresults are had by solution-treating fabricated products followingfabrication and then aging at the desired temperature. In this way thereis achieved a uniformity of product not had by aging immediatelyfollowing fabrication.

1n effecting the solution-treatment, especially after fabrication of thedesired products, a rather low solution-treating temperature is desiredthat is, about 1,400 to 1,500 F., and certainly not over l,750 F.,because excessive temperatures promote grain growth, this with resultingloss of toughness.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While the steel of my inventionin broadest aspect essentially consists of about 10.5 percent to about3.25 percent chromium or about 11.5 percent to about 13.25 percentchromium, about 7.5 percent to about 9.5 percent nickel, about 1 percentto about 2.5 percent molybdenum or about 1.3 percent to about 2.25percent molybdenum, about 1 percent to about 2.5 percent copper or about1 percent to about 2 percent copper, about 1 percent to about 2 percentaluminum or about 1.1 percent to about 1.8 percent aluminum, with carbonnot exceeding about 0.05 percent, manganese not exceeding about 0.10percent, silicon not exceeding about 0.10 percent, nitrogen notexceeding 0.015 percent, and remainder iron, a best combination ofproperties is had in steels of more limited composition. One such steelcontains about 1 1.50 percent to about 12.25 percent chromium, about8.40 percent to about 8.90 percent nickel, about 1.35 percent to about1.60 percent molybdenum, about 1 percent to about 1.5 percent copper,about 1.50 percent to about 1.75 percent aluminum, with carbon up toabout 0.05 percent and more particularly about 0.025 percent to about0.045 percent carbon, up to about 0.10 percent each of manganese andsilicon, up to 0.010 percent nitrogen, and remainder iron. Certainparticular benefits are had by including in the composition of the steelcolumbium in the amount of about 0.1 percent to about 0.2 percent, asnoted below.

A further steel according to my invention essentially consists of about1 1.85 percent to about 12.75 percent chromium, about 8.20 percent toabout 8.65 percent nickel, about 1.50 percent to about 1.85 percentmolybdenum, about 1 percent to about 1.5 percent copper, about 1.3percent to about 1.65 percent aluminum, with carbon not exceeding about0.05 percent and desirably about 0.025 percent to about 0.045 percent,with manganese and silicon each not exceeding about 0.10 percent andpreferably each not exceeding about 0.050 percent, with nitrogen notexceeding 0.010 percent and particularly not exceeding about 0.007percent, and remainder iron. Here again, columbium advantageously isincluded in the composition, this in amounts up to about 0.3 percent,especially about 0.1 percent to about 0.2 percent.

Another steel essentially contains about 12.5 percent to about 13percent chromium, about 8 percent to about 8.4 percent nickel, about1.75 percent to about 2 percent molybdenum, about 1 percent to about 1.5percent copper, about 1.15 percent to about 1.4 percent aluminum, withcarbon up to about 0.050 percent and more especially about 0.025 percentto about 0.045 percent, with manganese and silicon each not exceedingabout 0.10 percent particularly each of these two ingredients notexceeding about 0.05 percent, with nitrogen not exceeding 0.010 percentand particularly not exceeding about 0.007 percent, and remainder iron.Again, columbium advantageously is included in the composition, this inthe amount of about 0.1 percent to about 0.2 percent.

In my steel, the amounts of the ingredients chromium, nickel,molybdenum, copper and aluminum are in every sense critical, assuggested above; where there is any significant departure from thecompositional limits of these several ingredients, with either greateramounts than the upper limits set or lesser amounts than the set lowerlimits, one or more of the desired properties is lost or adverselyaffected. For example, with a chromium content less than the lowestpermissible limit of about 10.5 percent, corrosion-resistance suffers.So, also, there is a loss of strength. And with a chromium contentexceeding about 13.25 percent, the hardenability is adversely affected.With a nickel content less than about 7.5 percent, the steel is inclinedto contain an excessive amount of deltaferrite, hot-workability suffers,and strength, especially in direction transverse to working, is lost.Moreover, the steel is inclined to harden prematurely. Where the nickelcontent exceeds about 9.5 percent, the metal becomes too stable and doesnot readily lend itself to hardening by heat-treatment.

In my steel molybdenum is an essential ingredient. Not only does thisingredient lend a certain improvement to the corrosion-resistantcharacteristics of the metal, but it inhibits pitting. Additionally, itlends strength to the steel. And, most importantly, the molybdenumcontent inhibits overaging, that is, a sacrifice of the desired strengthwhere the hardening heattreatment is inadvertently conducted at somewhatexcessive temperatures.

Additionally, strength is assured by the copper content of the steel.Any tendency of overaging is further inhibited by the ingredientcolumbium where employed.

As particularly illustrative of the steel of my invention, 1 give belowin Table 1(a) a series of heat-hardenable chromium-nickel stainlesssteels, some of a composition according to TABLE 1(a) Chemicalcomposition of a series of chromium-nickel stainless steels Percent HeatNo, C Cr N1 M0 Cu Al Cb Tl N 1 Steel according to the invention.

Nora-Manganese less than 0.01%, silicon 0.05%, phosphorus 0.002%,sulphur 0.005%.

my invention in which a desired combination of properties is had, andfor comparative purposes, others not according to my invention of suchcomposition that one or more of the desired properties is lost; themechanical properties of the steels of Table 1(a) in the age-hardenedcondition are presented hereinafter in Table 1(b).

The mechanical properties of the steels of Table 1(a) are given below inTable I(b). These steels in the form of 30- pound ingots were soaked at2,150 F. for 4 hours and air cooled, equalized at 2,150 F. and forged to2 inch by 2 inch sections, hot-rolled to bar size, and cut for tensile,micro and impact samples. The samples were solution-treated at l,700 F.for one-half hour and quenched in oil, then age-hardened by heating at950 F. for 4 hours, followed by cooling in air. Tests were made oftensile strength in pounds per square inch, 0.2 percent yield strengthin pounds per square inch, percent reduction in area, percent elongationin 2 inches, Rockwell hardness on the C-scale, and Charpy V-notch impactstrength in foot-pounds.

1 Steel according to the invention.

Study of the test data presented above, this taken with the compositionof the steels reported on, clearly shows that those steels in whichcopper is absent (the Heat Nos. 153, 154 and 156) are of inadequatetensile strength. In each case the tensile strength is about 230,000psi, in any event significantly below the desired figure of 240,000 psi.With the copper addition, however (the heat No. 159), the desiredstrength is had. The impact strength developed in this steel is a bitlow for many applications. Copper, then, is seen to be an ingredientessential to my steel.

A further series of heat-hardenable chromium-nickel stainless steels ofsomewhat modified chemical composition is set out below in Table (0),with the properties of these further steels, aged at 900 and 950 F.,being presented in Table II(b).

TABLE II(a) Chemical composition of a furthetr series of chromium-nickelstainless s ee s Percent Heat No. C Cr Ni Mo Cu Al Cb N 1 Steelsaccording to the invention. 0 g$a.-Manganese 0.01%, silicon 0.02%,phosphorus 0.003%, sulphur Hard- Impact,

Aging P.s.1 Percent ness, ft.

Hea temp., 0- lbs. No. deg. F. Tensile Yield RA. Elong. Rock.

l Steels according to the invention. Premature failure.

It will be seen from Tables [1(a) and ll(b) that those chromium-nickelstainless steels containing about 13 percent chromi um and about 8percent nickel, with about 1.5 percent molybdenum and about 1 percentcopper (the heat No. 186), or containing about 2 percent molybdenum and2 percent copper (the beat No. 187) enjoy a good combination of strengthand ductility. But, here again, the impact resistance is low for someapplications. Whether aged at 900 or 950 F., the tensile strength ofboth steels is in excess of 240,000 psi and the elongation values exceed11 percent. In the steel of the 2 percent copper and 2 percentmolybdenum (the heat No. 187) there is had an impact strength sufficientfor most applications. The steel containing about 2 percent molybdenumand about 3 percent copper (the heat No. 188), however, is deficient instrength, this amounting to only some 220,000 psi when aged at 900 F.,and even less when aged at 950 F even though the steel enjoys goodimpact strength.

The chemical composition of five further chromium nickel stainlesssteels is given in Table lll(a) below:

TABLE lll(a) Chemical composition of an additional series 01chromium-nickel stainless stee s Steels according to the invention inwhich a best combination of properties is had.

I Steels according to the invention.

N0'rE.-Manganese 01%, silicon .01/02%, phosphorus 0.001%, sulphur.002/.004%.

The mechanical properties of the steels of Table lll(a) with samplesprepared as before but aged by heat-treatment at 1,000 F. for one setand at 1.050" F. for another, are set out below in Table lll(b).

TAB LE lll(b) Mechanical properties of the steels of Table lll(a) Hard-Aging P.s.1 Percent noss, Impact,

Heat temp., C- it No. deg. F. Tensile Yield R.A. Elong. Rock. lbs. 2061, 000 246,100 229, 000 47. 2 11.7 49. 5 12 206 1,050 226, 214, 600 54.1 13. 0 47. 5 35 207 1, 000 257, 700 236, 900 33.0 11. 5 49. 5 10 207 1,050 239, 200 225,000 50. 8 12.0 41). 5 10 208 1, 000 248, 000 231, 00044. 0 11.7 49. 5 5 208 1,050 224, 100 213,200 59. 1 13. 5 47. 5 13 209.1,000 234,300 230, 100 1 40. 5 4 209. 1, 050 225, 500 215, 500 3.2 3. 248.0 3 211 1,000 240,200 228,000 39. 4 10. 5 49. 0 5 211 L 1,050 225,200 212, 400 48. 1 11. 5 47. 0 7 1 Steals according to the invention inhich a best combination of properties is had.

Z Stee s according to the invention.

Now in studying the mechanical properties and chemical compositions ofthe steels reported in Tables lll(a) and lll(b), it will be seen thatthe steels containing about 12 percent chromium, about 8 percent nickel,about 1.5 percent molybdenum and about 1 percent copper (the heat Nos.206, 207 and 208) enjoy a good combination of strength, ductility andimpact resistance. Especially good results are had in the steels (heatNos. 206 and 207) which additionally contain columbium in the amount of0.15 percent. of these two, the one (beat No. 206) having an aluminumcontent of about 1.4 percent, while of lower tensile strength than thesteel having an aluminum content of about 1.6 percent (heat No. 207), ischaracterized by significantly higher impact strength and somewhatbetter ductility.

While the ingredient titanium may be effectively substituted forcolumbium in these steels, this substitution is accompanied by some lossin impact strength, as quickly may be seen by comparing the impactresistance values of the titanium-bearing steel (the heat 208), havingan impact strength of about ft.-lbs., with the columbium-bearing steel(the heat No. 206) having an impact strength of at least 12 ft.-lbs.,the tensile strengths of both steels being in excess of 240,000 psi whenaged at 1,000 F.

In all three of the steels, best results are achieved when aging is hadat 1,000 F.; the higher aging temperature of 1,050 F., althoughresulting in greater impact strength, gives a lower tensile strength.Particularly is this noted in the steel of the lower aluminum content(the heat No. 206), where the 50 difference in aging temperaturesresults in a difference of some 20,000 psi in tensile strength, eventhough the impact strength is just about tripled. A best balance oftensile strength and impact strength is had in the steel of the somewhathigher aluminum content (the heat No. 207), where the tensile strengthis consistently high and there is had good impact strength.

In my steel the ingredient molybdenum is considered to be essential, forwith molybdenum omitted, and even with copper increased and nickeldecreased to maintain the balance of the composition (heat No. 209),both tensile strength and impact strength directly suffer, whether agingis had at 1,000 or 1,050 F. And the ductility of the steel suffers evenmore than its impact strength.

Although under some circumstances the ingredient cobalt may besubstituted for a portion of the nickel content of my steel, thisordinarily is not recommended. As may be seen by comparing the steels ofHeat Nos. 21 1 and 186, both containing about 13 percent chromium, about1.5 percent molybdenum and about 1 percent copper, in which about 2percent cobalt in the heat No. 21 l is substituted for 2 percent of thenickel in heat No. 186, the cobalt-bearing steel, while of somewhatimproved impact strength, is somewhat lower in tensile strength. A bestcombination of strength and impact resistance is had in thecobalt-bearing steel by aging at 1,000 F.

Some eight additional chromium-nickel stainless steels of modifiedchemical composition are reported in Table 1V(a) below, these allcontaining about 12 percent chromium, 9 percent nickel, as well as theingredient columbium.

TABLE IV(a) Chemical com position of another series of chromium-nickelstainless steels Percent Heat No. C Cr Ni Mo Cu Al Cb N 243 l 035 12. 108. 78 2. 02 02 1. 28 .18 000 236 2 .031 12.13 0.17 1. 55 07 1. 50 .18007 237.... .025 12.23 8.75 .01 1.78 .17 .007 241.... .033 12.23 8. EH1.00 1.62 .16 .009 238.. .035 12.22 8. 88 .01 1.78 .10 .010 2-10. 03212. 11) T. 04 1. 51 3. 08 1. 00 18 005 211 .030 12.10 8.50 2.22 1.1-11.80 18 .005 24'. .030 12.33 8. 00 1. 50 .04 1. 53 .18 005 steelsaccording to the invention in which a best combination of properties ishnd.

Steels according to the invention.

Norm. Manganese .01/.02%, silicon .08/.14%, phosphorus .002/.003%,sulphur .005/.0025%.

Test samples of the eight steels of Table lV(a) were prepared as beforewith aging by heat-treatment at 1,000 and at l,050 F. The averagemechanical properties of duplicate samples of each for the two differingheat-treatments are reported below in Table lV(b).

TABLE IV(b) Mechanical properties of the steels 01 table IV(a) Hard-Aging P.s.i Percent ness, Impact,

Ileat temp., it No. deg. F. Tensile Yield R.A lbs.

243 1, 000 243, 300 223, 800 43. 0 13 243 1, 050 219, 206, 100 53. 8 46236 1, 000 240, 000 220, 300 35. 9 0 236. 1, 050 225, 600 212, 600 50. 22'.) 237 1 000 257, 000 236, 100 3 237 1, 050 232, 500 223, 000 30. 3 3244 1 000 "46, 200 230-800 3 244 1 050 227,000 215, 000 30. 0 3 238 1000 3 238 l 050 241. 300 230. 300 35. 4 3 240 1, 000 30, 000 210, 00042. 2 l5 2-10 l, 050 .200, 300 107. 700 52. 7 -15 241 l 000 .255, 00023., 000 41. T 7 -241 1, 050 227, 000 .214, 100 55.0 20 242 1, 000 255,700 233, 200 31. 0 -1 2 12 1, 050 220, 000 217, 700 51. 8 10 propertiesis bad.

1 Steels according to the invention.

Even in the steels containing about 12 percent chromium and about 9percent nickel, as may be seen from the test data presented immediatelyabove, molybdenum is a necessary and essential ingredient. In comparingthe steels containing about 12 percent chromium, about 9 percent nickeland about 1 percent copper, with some 1.3 to 1.8 percent aluminum, andfree of molybdenum (the heat Nos. 237, 244 and 238) with themolybdenum-bearing steels of like chromium, nickel, copper and aluminumcontents (the heat No. 243 with about 2 percent molybdenum and the heatNo. 236 with about 1.5 percent molybdenum), it will be seen that allthree of the steels which are free of molybdenum have rather poor impactvalues, this only amounting to about 3 ft.-lbs. Although the tensilestrengths of the steels aged at about 1,000 F. appear to be generallysatisfactory, it is the shortage in impact strength which makes thesesteels unacceptable.

The steels containing about 12 percent chromium, about 9 percent nickel,about 1 percent copper, with molybdenum about 1.5 percent (the heat Nos.236 and 242), molybdenum about 2 percent (the heat No. 243 andmolybdenum about 2.25 percent (the heat No. 241), when aged at about1,000 F., not only enjoy a tensile strength in excess of 240,000 psi,but an acceptable impact strength. The best combination of tensilestrength and impact strength is had in the steel containing molybdenumin the amount of about 2 percent (heat No. 243). The somewhat superiortensile strength had in the steel of beat No. 242 I attribute to theslightly higher chromium and aluminum contents and the slightly lowernickel and copper contents (12.33 percent chromium and 1.53 percentaluminum, with 8.96 percent nickel and 0.94 percent copper for the heatNo. 242 as compared to 12.13 percent chromium, 1.50 percent aluminum,with 9.17 percent nickel and 0.97 percent copper for the heat No. 236).

The steel having a chromium content of about 12 percent and a nickelcontent of only about 7.5 percent, with molybdenum about 1.5 percent andcopper about 3 percent, this in partial substitution for the lowerednickel content (heat No. 240) is not satisfactory. Although the impactstrength is high, the tensile strength is inadequate, falling as itdoes, significantly below 240,000 psi in the aged condition, even whereaging is had at 1,000 F.

In further illustration of a steel enjoying a best combination ofstrength, ductility and impact resistance, there was melted a steelaccording to the specification given in Table V(a) below, there alsobeing indicated the actual chemical analysis of the steel.

TABLE V(a) Chemical composition of a best steel according to theinvention Percent C Cr N 1 Mo Cu Al Cb N Specification .025/.04511.50/12. 25 8. 40/800 1.35/l.60 1.00/1.50 1.50/1.75 .12/. 17 .010 HeatNo. (60689) .036 12.07 8.81 1.61 1.21 1.04 .20 0.000

1 Maximum Manganese 0.01%, silicon 0.05 phosphorus 0.007%, sulphur0.003%. 2 Steel according to the invention in which a best combinationof properties is had.

L Tensile specimens and impact specimens were cut from the steel in theform of Va inch diameter hot-rolled bar, duplicate samples beingsolution-treated at 1,500, 1,600 and 1,700 F., oil quenched, machinedand aged with particular aging as given in Table V(b) below reporting ontensile strength in pounds per square inch, 0.2 percent yield strengthin pounds per square inch, percent reduction in area, percent elongationin four times the diameter, and Charpy V-notch impact strength infoot-pounds.

TABLE V(b) Mechanical Properties of the Steel of Table V(a) forDiffering Solution-Treating Temperatures Sample Tensile Yield Impact No.R.A. Elong. psi psi Ft.-Lbs. 1 46.7 9.3 262,300 249,700 7 Samples 1 and2 l,500 F. for 1% hours; oil quench;

machine; 1,000 F. for 4 hours; air cool.

Samples 3 and 4 1,600 F. for 1% hours; oil quench;

machine; 1,000 F. for 4 hours; air cool.

Samples 5 and 6 l,700 F. for 1% hours; oil quench and cool to 60 F.;machine; 1,000 F. for 2 hours; cool to room temperature; 1,000 F. for 2hours; water quench.

From the results presented immediately above, it will be seen that thereis had an excellent combination of' tensile strength, ductility andimpact strength. While somewhat greater strength is achieved with asolution-treating temperature of 1,500 F good strength nevertheless ishad where the steel is solution-treated at 1,600 F., or even 1,700 F. Inpoint of fact, somewhat greater ductility and impact strength are hadwith the l,700 F. solution-treating temperature. In every case, aging ishad at 1,000 F. whether for 4 hours continuously or two agings at 2hours each. The tensile strength in all instances is well above 240,000psi; indeed, for samples aged following solution-treatments at 1,500 and1,600 F., even the yield strength exceeds 240,000 psi.

In my steel some improvement is had by increasing the duration of theaging treatment beyond about 4 hours. Duplicate samples of the steel ofheat No. 60689, solution-treated at l,500 F. for 1 hour, oil quenched,machined and aged at 1,000 F. for 8 hours, and for 12 hours, and aircooled, were tested, with the results set out below in Table V(c).

Samples 11 and 12 l,500 F. for 1 hour; 011 quench;

machine; 1,000 F. for 8 hours; air cool. Samples 13 and 14 l,500 F. for1 hour; oil quench;

machine; 1,000 F. for 12 hours; air cool. As indicated, some benefit ishad by the prolonged aging. With the prolonged aging, the strength fallsoff a bit, as may be seen by comparing the mechanical properties of thesamples ll-12and 13-14 given above with the samples 1-2 of the TableV(b). But with the loss of strength, that is, from an average of 262,800psi for samples 1-2 to an average of 254,500 psi for samples 11-12, anddown to 248,700 psi for samples 13-14, there is some improvement inductility, the elongation increasing from an average of 9.6 percent forthe samples l-2, to 11.4 percent for the samples 11-12 and 13-14, andthe impact strength is increased, thisfrom some 7 or 8 ft.-lbs. to some13 or 14 ft.-lbs.

In conclusion, it will be seen that I provide in my inventionaprecipitation-hardenable chromium-nickel-molybdenumcopper stainlesssteel in which there are had the various objects of my invention, andthe many advantages thereof, as more particularly set out above. In mysteelthere is enjoyed a combination of great strength in theage-hardened condition. that is, a strength exceeding 240,000 psi,together with toughness and impact strength.

My steel works well'both in the hot-mill and in the cold-mill, readilylends itself to a variety of forming and fabricating operations, such asmachining, drawing, rolling, upsetting, and the like, following whichthe steel may be hardened by simple heat-treatment, that is, by mereaging at moderate temperatures, to achieve a combination of strength andtoughness.

Actually, in the production of machine screws and other fasteners I findthat maximum uniformity in properties is achieved, as suggested above,by reheating the fasteners to solution-treating temperature (some 1,400to l,750 F.) and then aging the same, this at the 900 to 1,050 P.temperature. This treatment conveniently is handled as a batchoperation. The solution-treating step alleviates the stresses introducedin manufacture. And with the subsequent aging treatment there are hadproperties which are uniform throughout the batch.

Inasmuch as there are many embodiments which may be made of myinvention, and numerous changes made in the embodiments hereinbefore setforth, it is to be understood that all matter described herein is to beinterpreted as illustrative and not by way of limitation.

Iclaim:

1. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 10.5 percent to about 13.25 percentchromium, about 7.5 percent to about 9.5 percent nickel, about 1 percentto about 2.5 percent molybdenum, about 1 percent to about 2.5 percentcopper, about 1 percent to about 2 percent aluminum, carbon notexceeding about 0.05 percent, nitrogen not exceeding 0.015 percent, andremainder substantially all iron.

2. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 10.5 percent to about 12.5 percentchromium, about 8 percent to about 9.5 percent nickel, about 1.3 percentto about 2.25 percent molybdenum, about 1 percent to about 2.5 percentcopper, about 1.1 percent to about 1.8 percent aluminum, carbon notexceeding about 0.05 percent, nitrogen not exceeding about 0.01 percent,and remainder substantially all iron.

. 3. Precipitation-hardenable martensitic chromium-nickel stainlesssteel essentially consisting of about 11.5 percent to about 13 percentchromium, about 7.5 percent to about 9 percent nickel, about l.3 percentto about 2.25 percent molybdenum, about 1 percent to about 2.5 percentcopper, about 1.1 percent to about 1.8 percent aluminum, with at leastone ingredient of the group consisting of columbium up to about 0.3percent and titanium up to about 0.15 percent, carbon not exceedingabout 0.05 percent nitrogen not exceeding .01 percent, and remaindersubstantially all iron.

4. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 11.85 percent to about 12.75 percentchromium, about 8.20 percent to about 8.65 percent nickel, about 1.50percent to about 1.85 percent molybdenum, about 1 percent to about 1.5percent copper, about 1.5 percent to about 1.65 percent aluminum, up toabout 0.3 percent columbium, carbon not exceeding about 0.05 percent,nitrogen not exceeding 0.01 percent, and remainder substantially alliron.

5. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 11.50 percent to about 12.25 percentchromium, about 8.40 percent to about 8.90 percent nickel, about 1.35percent to about 1.60 percent molybdenum, about 1 percent to about 1.5percent copper, about 1.50 percent to about 1.75 percent aluminum,ab0ut0.1 percent to about 0.2, percent columbium, about 0.025 percent toabout .045 percent carbon, nitrogen not exceeding 0.010

percent, and remainder substantially all iron.

6. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 12.5 percent to about 13 percentchromium, about 8 percent to about 8.4 percent nickel, about 1.75percent to about 2 percent molybdenum, about 1.4 percent to about 1.5percent copper, about 1.1 percent to about 1.4 percent aluminum, about0.025 percent to about 0.045 percent carbon, manganese and silicon eachnot exceeding about 0.05 percent, nitrogen not exceeding 0.010 percent,and remainder substantially all iron.

7. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 11.5 percent to about 13.25 percentchromium, about 7.5 percent to about 9 percent nickel and cobalt takentogether with cobalt being up to about 2 percent and with nickel beingat least 6 percent, about 1 percent to about 2.5 percent molybdenum,about 1 percent to about 2.5 percent copper, about 1 percent to about 2percent aluminum, carbon not exceeding 0.05 percent, nitrogen notexceeding 0.015 percent, and remainder substantially all iron.

8. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 11.5 percent to about 13.25 percentchromium, about 6.5 percent to about 9 percent nickel, about 1 percentto about 2 percent cobalt,

about 1 percent to about 2.5 percent molybdenum, about 1 percent toabout 2.5 percent copper, about 1 percent to about 2 percent aluminum,carbon not exceeding about 0.05 percent nitrogen not exceeding 001percent, and remainder substantially all iron.

9. Precipitation-hardenable martensitic chromium-nickel stainless steelflat-rolled products essentially consisting of about 11.5 percent toabout 13.25 percent chromium, about 7.5 percent to about 9 percentnickel, about 1 percent to about 2.5 percent molybdenum, about 1 percentto about 2.5 percent copper, about 1.1 percent to about 1.8 percentaluminum, up to about 0.3 percent columbium, carbon not exceeding about0.05 percent, nitrogen not exceeding 0.01 percent, and remaindersubstantially all iron.

10. Precipitation-hardenable martensitic chromium-nickel stainless steelforgings essentially consisting of about 1 1.5 percent to about 13percent chromium, about 7.5 percent to about 9 percent nickel, about 1percent to about 2.25 percent molybdenum, about 1 percent to about 2.5percent copper. about 1.1 percent to about 1.8 percent aluminum, withcarbon not exceeding about 0.05 percent, manganese and silicon each notexceeding about 0.10 percent, nitrogen not exceeding 0.01 percent, andremainder substantially all iron.

11. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 10.5 percent to about 13.25 percentchromium, about 7.5 percent to about 9.5 percent nickel, about 1 percentto about 2.5 percent molybdenum, about 1 percent to about 2.5 percentcopper, about 1 percent to about 2 percent aluminum, carbon notexceeding about 0.05 percent, manganese and silicon each not exceedingabout 0.10 percent, nitrogen not exceeding 0.015 percent, and remaindersubstantially all iron.

12. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 11.5 percent to about 13 percentchromium, about 7.5 percent to about 9 percent nickel, about 1.3 percentto about 2.25 percent molybdenum, about 1 percent to about 2.5 percentcopper, about 1.1 percent to about 1.8 percent aluminum, up to about 0.3percent columbium, with a carbon content not exceeding about 0.05percent, nitrogen not exceeding 0.01 percent, and

remainder substantially all iron.

13. Precipitation-hardenable martensitic stainless steel essentiallyconsisting of 9 11.5 percent to about 13 percent chromium, about 8percent to about 9 percent nickel, about 1.3 percent to about 2 percentmolybdenum, about 1 percent to about 1.5 percent copper, about 1.1percent to about 1.4 percent aluminum, with columbium up to about 0.3percent, about 0.02 percent to about 0.05 percent carbon, with manganeseand silicon each not exceeding about 0.10 percent,

nitrogen not exceeding 0.007 percent, and remainder substantially alliron.

2. Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 10.5 percent to about 12.5 percentchromium, about 8 percent to about 9.5 percent nickel, about 1.3 percentto about 2.25 percent molybdenum, about 1 percent to about 2.5 percentcopper, about 1.1 percent to about 1.8 percent aluminum, carbon notexceeding about 0.05 percent, nitrogen not exceeding about 0.01 percent,and remainder substantially all iron.
 3. Precipitation-hardenablemartensitic chromium-nickel stainless steel essentially consisting ofabout 11.5 percent to about 13 percent chromium, about 7.5 percent toabout 9 percent nickel, about 1.3 percent to about 2.25 percentmolybdenum, about 1 percent to about 2.5 Therefore percent copper, about1.1 percent to about 1.8 percent aluminum, with at least one ingredientof the group consisting of columbium up to about 0.3 percent andtitanium up to about 0.15 percent, carbon not exceeding about 0.05percent nitrogen not exceeding .01 percent, and remainder substantiallyall iron.
 4. Precipitation-hardenable martensitic chromium-nickelstainless steel essentially consisting of about 11.85 percent to about12.75 percent chromium, about 8.20 percent to about 8.65 percent nickel,about 1.50 percent to about 1.85 percent molybdenum, about 1 percent toabout 1.5 percent copper, about 1.5 percent to about 1.65 percentaluminum, up to about 0.3 percent columbium, carbon not exceeding about0.05 percent, nitrogen not exceeding 0.01 percent, and remaindersubstantially all iron.
 5. Precipitation-hardenable martensiticchromium-nickel stainless steel essentially consisting of about 11.50percent to about 12.25 percent chromium, about 8.40 percent to about8.90 percent nickel, about 1.35 percent to about 1.60 percentmolybdenum, about 1 percent to about 1.5 percent copper, about 1.50percent to about 1.75 percent aluminum, about 0.1 percent to about 0.2percent columbium, about 0.025 percent to about .045 percent carbon,nitrogen not exceeding 0.010 percent, and remainder substantially alliron.
 6. Precipitation-hardenable martensitic chromium-nickel stainlesssteel essentially consisting of about 12.5 percent to about 13 percentchromium, about 8 percent to about 8.4 percent nickel, about 1.75percent to about 2 percent molybdenum, about 1.4 percent to about 1.5percent copper, about 1.1 percent to about 1.4 percent aluminum, about0.025 percent to about 0.045 percent carbon, manganese and silicon eachnot exceeding about 0.05 percent, nitrogen not exceeding 0.010 percent,and remainder substantially all iron.
 7. Precipitation-hardenablemartensitic chromium-nickel stainless steel essentially consisting ofabout 11.5 percent to about 13.25 percent chromium, about 7.5 percent toabout 9 percent nickel and cobalt taken together with cobalt being up toabout 2 percent and with nickel being at least 6 percent, about 1percent to about 2.5 percent molybdenum, about 1 percent to about 2.5percent copper, about 1 percent to about 2 percent aluminum, carbon notexceeding 0.05 percent, nitrogen not exceeding 0.015 percent, andremainder substantially all iron.
 8. Precipitation-hardenablemartensitic chromium-nickel stainless steel essentially consisting ofabout 11.5 percent to about 13.25 percent chromium, about 6.5 percent toabout 9 percent nickel, about 1 percent to about 2 percent cobalt, about1 percent to about 2.5 percent molybdenum, about 1 percent to about 2.5perCent copper, about 1 percent to about 2 percent aluminum, carbon notexceeding about 0.05 percent nitrogen not exceeding 0.01 percent, andremainder substantially all iron.
 9. Precipitation-hardenablemartensitic chromium-nickel stainless steel flat-rolled productsessentially consisting of about 11.5 percent to about 13.25 percentchromium, about 7.5 percent to about 9 percent nickel, about 1 percentto about 2.5 percent molybdenum, about 1 percent to about 2.5 percentcopper, about 1.1 percent to about 1.8 percent aluminum, up to about 0.3percent columbium, carbon not exceeding about 0.05 percent, nitrogen notexceeding 0.01 percent, and remainder substantially all iron. 10.Precipitation-hardenable martensitic chromium-nickel stainless steelforgings essentially consisting of about 11.5 percent to about 13percent chromium, about 7.5 percent to about 9 percent nickel, about 1percent to about 2.25 percent molybdenum, about 1 percent to about 2.5percent copper, about 1.1 percent to about 1.8 percent aluminum, withcarbon not exceeding about 0.05 percent, manganese and silicon each notexceeding about 0.10 percent, nitrogen not exceeding 0.01 percent, andremainder substantially all iron.
 11. Precipitation-hardenablemartensitic chromium-nickel stainless steel essentially consisting ofabout 10.5 percent to about 13.25 percent chromium, about 7.5 percent toabout 9.5 percent nickel, about 1 percent to about 2.5 percentmolybdenum, about 1 percent to about 2.5 percent copper, about 1 percentto about 2 percent aluminum, carbon not exceeding about 0.05 percent,manganese and silicon each not exceeding about 0.10 percent, nitrogennot exceeding 0.015 percent, and remainder substantially all iron. 12.Precipitation-hardenable martensitic chromium-nickel stainless steelessentially consisting of about 11.5 percent to about 13 percentchromium, about 7.5 percent to about 9 percent nickel, about 1.3 percentto about 2.25 percent molybdenum, about 1 percent to about 2.5 percentcopper, about 1.1 percent to about 1.8 percent aluminum, up to about 0.3percent columbium, with a carbon content not exceeding about 0.05percent, nitrogen not exceeding 0.01 percent, and remaindersubstantially all iron.
 13. Precipitation-hardenable martensiticstainless steel essentially consisting of 9 11.5 percent to about 13percent chromium, about 8 percent to about 9 percent nickel, about 1.3percent to about 2 percent molybdenum, about 1 percent to about 1.5percent copper, about 1.1 percent to about 1.4 percent aluminum, withcolumbium up to about 0.3 percent, about 0.02 percent to about 0.05percent carbon, with manganese and silicon each not exceeding about 0.10percent, nitrogen not exceeding 0.007 percent, and remaindersubstantially all iron.